Means for adjusting sensitivity of spiral path scanning mechanism

ABSTRACT

In spiral scanning apparatus for bubble chamber photographs, comprising an elongated plane mirror which is lengthwise inclined upwardly and outwardly to an axis about which it rotates, to reflect toward the axis light shone downwardly onto it, and a periscope which rotates to have its eye face the mirror and simultaneously moves axially to scan along it, superimposed slotted fixed and movable mask means in front of the periscope eye enable sensitivity and selectivity to be adjusted for different photographs.

SEARCH ROOM 112 mevsmvs 1 I K v e 5 v A 111 3,573,475

[ 1 Invent Nmdlund; 501 Field of Search 250/219 Bengt Wetterbrandt, Jonkoplng, Sweden (IA), 236 237; 3 50 /7 21 Appl. No. 853,830 [22] Filed Aug. 28, 1969 [56] References Cited Patented 2 2: 13 UNITED STATES PATENTS [73 Assignee aa tie Linkoping Sweden 3,421,010 1/1969 Toby 250/219QA [32] Priority Aug. 28, 1968 Primary Examiner-James W. Lawrence [33] Sweden Assistant Examiner-T. N. Grigsby [31 11533168 Attorney-Ira Milton Jones ABSTRACT: In spiral scanning apparatus for bubble chamber photographs, comprising an elongated plane mirror which is lengthwise inclined upwardly and outwardly to an axis about which it rotates, to reflect toward the axis light shone [54] MEANS FOR ADJUSTING SENSITIVITY OF SPIRAL PATH SCANNING MECHANISM 5 Clams 5 Drawing Flgs' downwardly onto it, and a periscope which rotates to have its [52] US. Cl 250/219, eye face the mirror and simultaneously moves axially to scan 350/7 along it, superimposed slotted fixed and movable mask means [5 l] Int. Cl G01n 31/30, in front of the periscope eye enable sensitivity and selectivity ooze 17/00, 110 i j 3/14 to be adjusted for different photographs.

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Patente April 6, 1971 4 Sheets-Sheet 1 rbrandb ATTONEY 4 Sheets-Sheet 2 lllllllllIlllHHIHIIIIHIIIHIII INVENTORS 521221 Nordzunc] B221 2; Wefiebranut w ATTORNEY Patented April 6, 1971 4 Sheets-Sheet 4 IGA.

FIG-.5.

,I'III) a. 1 mm A 0 n/p/d INVENTORS 517211 Nurdlund Bengt milberbrandb ATTORNE MEANS FOR ADJUSTING SENSITIVITY OF SPIRAL PATH SCANNING MECHANISM This invention relates generally to apparatus for scanning photographs of the tracks made by charged particles in a bubble chamber or cloud chamber for the purpbse of converting the records contained on such photographs to signals of a form that can be used by a computer and ultimately analyzed for obtaining information about particle interactions and decays in nuclear physics experiments; and the invention relates more particularly to means for adjustably varying the selectivity and sensitivity of a so-called spiral reader for such photographs in accordance with density, contrastiness and other characteristics of photographs to be scanned.

A bubble or cloud chamber containing fluid in a condition of equilibrium between the liquid and gaseous states andsurrounded by a superconductive electromagnet, is a well kno'wn device used in nuclear physics experiments. When a charged particle is shot into the chamber, it leaves a track of tinybubbles; and when it encounters another particle (which en-' counter is called an event") the particles taking part in the event trace branching tracks which diverge from the point of the encounter. These bubble tracks, although persisting only very briefly, can be photographed stereoscopically to provide a record of the several paths taken by the particles involved in each event. Since the motions of the particles provide a clue to their natures, the study of photographic records of the branching or diverging tracks that denote events is of particular interest. 4

A reading machine of the type to which this invention relates is used with an electronic computer to scan bubble chamber photographs for the purpose of converting the pic torial records of events to a form of data that can be stored in and used by the computer. When a sufficient amount of such data is available, derived from the photographic recordsof numerous experiments, conclusions can be drawn, on the basis of statistical analysis of the data, aboutthe nature and charac'- teristics of the particles involved in the events being studied,

Inasmuch as valid conclusions can be drawn only on the basis of statistical study of a very large number of experiments, a tremendous amount of film needs to be analyzedtruckloads, in many cases. Automation of the reading of the film records is therefore essential to reasonable management and utilization of the data contained in such records.

Each frame of film to a be analyzed may contain one or more sets of diverging tracks that signify events, along with numerous other lines that trace the paths of particles that did not take part in events and which are therefore not of interest. Selection of each part of a frame of film to be scanned is accomplished, at least in part, with the aid of a human operator. As it scans the image on the film, the automatic reader produces signals corresponding to all tracks on the portion of the film being scanned. These signals are fed to a computer which in efiect rejects signals corresponding to tracks not of interest and preserves data relating to events selected for analysis. v The nature of the tracks to be analyzed is such that the most efficient and useful pattern of scanning is a spiral one in which the origin of the spiral is at the point where several diverging tracks have their junction or vertex that denotes an event. Mechanism for effecting scanning in a spiral path is disclosed in the companion application of Sten Norland et al., Ser. No. 853,244, filed Aug. 27, 1969. In general such mechanism comprises an elongated plane mirror which is carried for revolution in an orbit, the mirror being lengthwise inclined to its axis of rotation and being oriented to receive light from one direction along said axis and to reflect it toward said axis. Cooperating with .the rotating mirror is a periscope which rotates with the mirror so that its eye faces the mirror at all times and which also moves parallel to said axis so as to scan along the length of the mirror. A light responsive transducer such as a photoelectric cell is operatively associated with the periscope to issue signals which correspond to the changing values of light falling upon the eye of the periscope.

The present invention is based upon a recognition that the effective shape and area of the periscope eye can materially affect the results obtained from scanning. ln general, the periscope eye should have an aperture which is elongated in the direction of the axis about which the mirror rotates, so that the scanning mechanism is most responsive to lines which radiate from a center that coincides with that axis (as do the tracks which denote an event) and is less responsive to lines which all extend generally in one direction (as do the tracks of charged particles which do not take part in events). However, the relative strength of signals corresponding .to event tracks and to other tracks will depend upon the width of the slot that defines the periscope eye aperture. Where a film has relatively few tracks not of interest, or has good contrastiness, a relatively wide slot may be desirable to provide a high level of sensitivity; whereas with a low contrast film a relatively narrow slot may be needed to provide a high level of discrimination. The signal from the light responsive transducer can be manipulated accordingly, by varying the gain factor more or less in inverse ratio to the area of the slot being used and by filtering out signals which are below a predetermined level.

Thus it is a general object of this invention to provide a mirror and periscope type of scanning mechanism, particularly for the reading of bubble chamber photographs, having means for varying sensitivity and selectivity of the scanning mechanism in accordance with characteristics of a photograph to be scanned.

More particularly, it is an object of this invention to provide readily adjustable means for varyingthe effective area of the eye of the periscope in such apparatus.

In a specific sense, it an object of this in invention to provide, in a scanning device comprising a rotating and axially movable periscope, means for varying the effective area of the periscope eye, comprising a fixed mask and an adjustable mask which are in superimposed relationship with one another and with the periscope eye and which cooperate to define a slot of a desired width that forms the opening of the periscope eye; and means for readily adjusting the adjustable mask to provide the desired width of slot.

With these observations and objects in mind, the manner in which the invention achieves its purpose will be appreciated from the following description and the accompanying drawings. This disclosure is intended merely to exemplify the invention. The invention is not limited to the particular struc-' ture disclosed, and changes can be made therein which lie within the scope of the appended claims without departing from the invention.

The drawings illustrate one complete example of a physical embodiment of the invention constructed according to the best mode so far devised for the practical application of th principles thereof, and in which:

FIG. 1 is a general perspective view of reading apparatus embodying the principles of this invention, shown in operative relation to other apparatus with which it cooperates; h FIG. 2 isa vertical sectional view through spiral scanning apparatus embodying this invention;

iFlG. 3 is a sectional view on an enlarged scale taken on the plane ofnie line 3-3 in FIG. 2;

FIG; 4 is a perspective view of the mechanism for varying the effective area of the periscope eye; and

FIG. 5 is an enlarged perspective view of the movable prisni comprising the periscope eye, particularly illustrating the movable mask means thereon.

7 Referring now more particularly to the accompanying drawings, the numeral 4 designates generally a reading machine in which the present invention is embodied, and FIG.

1 illustrates it in its relationship to apparatus with which it cooperates.

The film to be analyzed or read by means of the reading machine 4 is in the form of long strips, each divided along its length into a numberof frames, and each frame usually con taining a record of one or more events which are of interest. To facilitate handling, each strip is normally wound onto a reel to form a roll. Inasmuch as each experiment in a bubble chamber is photographed simultaneously by three cameras, focused along coordinate axes, the three related rolls of film must be read more or less simultaneously, to coordinate data for each frame in each roll with that for the corresponding frames in the other two rolls.

In FIG. 1, three related strips of film to be scanned by the reading machine are respectively designated 5, 6 and 7. They extend across a horizontal table 8 on the reading machine having three apertures, one for each roll, that can be brought into alignment with a suitable light source (not shown) which is located beneath the table. Film transport means, comprising a suitable capstan drive 9, provides for advancing the film strips simultaneously across the table 8, to permit successive frames on each strip to be brought into register with the apertures in the table for the purpose of scanning. In addition, the film table is capable of bodily movement in all horizontal directions to permit any selected portion of a frame of film to be brought into alignment with the axis of an objective head 10 which is fixed above the film table and which comprises reflecting means. A companion application, Ser. No. 854,147 filed Aug. 28, 1969, discloses means for constraining the film takeup and supply reels to move horizontally with the table, to avoid twisting distortion of the film.

The image on the film frame that is under the objective head is projected up into the objective head, thence horizontally in one direction, and finally down onto the top surface of an operators table 11. An obliquely disposed mirror 12 above the operator's table receives the image from the objective head and reflects it downwardly onto the operators table.

The image is also projected from the objective head in the opposite horizontal direction, toward a half-silvered mirror 13, whence it is reflected downwardly into scanning mechanism 14 which is described hereinafter. The image is also projected through the half-silvered mirror into a television camera 15 which receives only a very small portion of the total image on a film frame, corresponding to about a square millimeter of the film around the axis of the objective head. The image of this small area is presented to the operator, in very greatly magnified form, on the screen of a television receiver 16 that is connected with the television camera. This highly magnified portion of the total image is used for accurately locating the film table to bring the vertex of the event record to be studied into exact coincidence with the axis of the objective head and hence into coincidence with the scanning axis of the scanning device 14.

On the operator's table there is a control console 17 for controlling film advance and the horizontal movements of the film table. The operator also has a typewriter 19 for input and output of data and an oscilloscope 20 that gives a graphic representation of light values measured during scanning.

The scanning device 14 is connected with an electronic computer 21 through an adapter 22 which is in itself a form of electronic computer.

The image of an event on a film frame is scanned in a spiral path, and hence the scanning apparatus is known as a spiral reader. The scanning begins at the vertex, progressing outwardly and around. Each frame of film to be scanned is established in a position in which the vertex of an event record thereon coincides with the axis of the scanning mechanism. Such positioning of the film frame is effected by appropriate movement of the film table 8, partly under the control of the operator and partly under that of the computer.

As scanning progresses, the scanning mechanism produces a signal each time a track on the film record is crossed. Since data concerning the radial and angular coordinates of the scan are continuously fed to the computer, the scanning process results in the storing of data which characterizes the event.

After the record of a selected event on a frame of film has been scanned, records of the same event on the counterpart frames of the other two rolls of film are similarly scanned, the vertices of the other two records being automatically located by the computer and aligned with the scanning axis. Data from the three scannings of the event are combined by the computer into a form that defines an unambiguous three dimensional curve which characterizes the event, and this is stored in the computer to be available for comparison with similarly obtained data for other events, for the purposes of statistical analysis.

The scanning device 14 by which the photographed image of an event is scanned along the desired spiral path comprises, in general, concentrically arranged inner and outer members 25 and 26. The outer member 26 carries a plane mirror 27 and is constrained to rotation on a fixed axis. The inner member 25 comprises a periscope and is constrained to both rotation with the outer member and motion relative to it in directions accurately parallel to said fixed axis.

Because of the positioning of the film frame being scanned, as described above, the axis about which the mirror carrying outer member 26 rotates can be considered to coincide with the vertex of the event being scanned.

The outer member comprises a generally funnel shaped body 29, as seen in FIG. 2, which is preferably of cast iron to insure rigidity. The mirror 27, which is elongated and generally rectangular, is secured in a groove in the inner surface of the conical upper portion of the funnel-shaped body. The mirror extends lengthwise in the direction radial to the rotational axis (as best seen in FIG. 3), and it is disposed obliquely to that axis, being inclined, along its length, radially outwardly and upwardly (as shown in FIG. 2), so that it reflects inwardly toward the axis light projected down onto it parallel to the axis. Thus as the mirror is carried orbitally in consequence of rotation of the outer member, it defines a reflecting cone and scans a circular area of the film frame that has at its center a vertex denoting an event. At every instant during such scan the mirror reflects toward the axis a striplike radially extending portion of that circular area. Note that the bottom portion of the mirror extends across the rotational axis, so that the strip reflected by the mirror will always include the vertex of the event.

As the inner periscope member 25 rotates with the mirror, its eye always faces the mirror. Hence, as the periscope moves upwardly parallel to the axis of rotation, it scans along the strip being scanned by the mirror. And since that strip is moving angularly around the vertex at a steady rate, and the periscope is scanning at a steady rate along that strip, the net effect of the combined scanning motions is the desired spiral scan. A companion application, Ser. No. 853,242, filed Aug. 27, 1969 discloses means for constraining the periscope to rotation with the mirror and to simultaneous scanning motion accurately parallel to the axis about which the scanning mechanism rotates.

The lower tubular or spout portion of the funnel-shaped body member 29 is journaled in a pair of axially spaced apart angular contact ball bearings 30.

The mirror carrying member is driven in rotation by an electric motor 36 which is mounted to one side of it and which is connected with it by means of a belt 38 and a drive pulley 39 on the motor. The belt is received in a circumferential belt groove 40 around the conical top portion of the mirror carrying member, spaced a small distance above the upper ball bearing 30.

An angle transducer 41 is operatively associated with the mirror carrying member to issue signals denoting its position of rotation, which signals are utilized by the computer in its processing of scanning information.

A coverlike member 42 which is secured to the top of the funnel-shaped member serves the dual function of providing great rigidity to the mirror carrying member, so that it resists mirror displacing distortion due to centrifugal force, and steadying the inner periscope member to prevent it from whipping or wobbling. The coverlike member 42 has a central hublike portion 43 through which the inner periscope member 25 extends for steadying and guidance and has a radially extending slot 44 which opens from the hub portion and which is located directly above the mirror, through which light projected from the objective head passes downwardly to the mirror.

The inner periscope member 25 comprises a rigid, elongated body member 46 which is substantially U-shaped in cross section along most of its length, to define a slotlike shaft 45 down which light can pass to the mirror 27. The optical elements of the periscope comprise a fibre optic rod 48 which transmits light along its length and a small prism or reflector 49 on the top of the fibre optic rod which receives light from the mirror 27 and reflects it down into the rod 48.

The leg portions 50 of the U-shaped body member 46 straddle the mirror 27 with a small clearance, as best seen in FIG. 3. The fibre optic rod 48 is secured in a small lengthwise extending recess or groove in the rear wall surface of the light shaft 45 in the body member 46.

The periscope comprising the body member 46, the prism or reflector 49 and the fibre optic rod 48 is moved up and down by means of a reversible electric drive mechanism 51 which is mounted beneath the rotating members and which is connected with the lower portion of the periscope body member by means of a connecting shaft 56. The upper end portion of the connecting shaft 56 is connected with the lower end portion of the periscope body by means of a suitable axial thrust bearing 57 which provides for rotation of the periscope body relative to the connecting shaft but constrains the periscope to axial motion with it.

A position responsive transducer 58 that is associated with the axial drive mechanism feeds signals to the computer that denote the axial position of the periscope.

The prism or reflector 49 is necessarily eccentric to the axis about which the inner and outer members rotate, inasmuch as the mirror extends across that axis, and the fibre optic rod 48 must likewise be spaced from that axis along most of its length. Near its bottom, however, the fibre optic rod has an offset bend, as at 60, so that its bottom end is on the axis and is located in or near the bearing connection between the periscope body member 46 and the connecting shaft 56. Another length of fibre optic, designated 61, extends through the upper portion of the connecting shaft 56 (which is tubular to accommodate it) with its upper end closely adjacent to the bottom end of the rotating fibre optic rod 48. The lower nonrotating fibre optic rod extends laterally out of the connecting shaft through a side aperture in the latter, and over to a light responsive transducer or photoelectric cell 62 which is mounted in a fixed position near the scanning mechanism. The flexibility of the fibre optic accommodates the up and down motion of the connecting shaft 56 relative to the transducer 62. It has been found that light losses across the joint between the two lengths of fibre optic are not significant.

In accordance with the principles of this invention, means are provided for adjustably varying the selectivity and sensitivity of the periscope, to adapt the scanning mechanism for the most effective reading of photographs which differ in density, contrastiness and other characteristics. Such adjustment means comprises, in general, a pair of cooperating apertured masks 70 and 71, one fixed and the other movable, the two masks being mounted in flatwise superimposed relationship, one behind the other, at the focal plane of the objective [0 and defining the aperture of the periscope eye. The movable mask is apertured to provide a plurality of slots 72a, 72b, 72c, 72d that are of different widths and/or heights, and it can be adjusted to bring any one of those slots into register with a single slot 73 in the stationary mask.

As shown in FIGS. 3 and 4, the stationary mask 70 comprises an upstanding flat projection on a holder 74 that is anchored to the periscope body member, while the movable mask 71 flatwise overlies and is directly secured to the front face of the periscope prism 49, which is mounted for movement in directions transverse to the axis about which the mirror rotates and edgewise of the masks.

The holder 74 has a generally tubular bottom portion 75 in which the upper end portion of the fibre optic rod 48 is snugly received and from which flanges 76 project in opposite directions to flatwise overlie the rear wall surface of the light shaft on opposite sides of the groove for the fibre optic rod. Screws 77 extending through the flanges 76 and threaded into the periscope body member securely anchor the holder to that body member. In addition to supporting the stationary mask 70, the holder 74 thus serves to confine the upper end of the fibre optic rod against lateral motion relative to the periscope body, thereby preventing whipping and swaying of the rod.

The aperture 73 in the stationary mask is a slot which is elongated in the direction of the axis of rotation of the periscope and mirror and which has its longitudinal centerline on a plane lying on that axis and perpendicular to the mirror.

The movable mask 71 can comprise a thin metal shim or a metal coating which can be applied to the front face of the prism by evaporative deposition. The slots 72a 72d in it are of different widths, ranging, for example, from 1/10 mm. to 1 mm., and the area scanned by the prism or periscope eye depends upon which one of those slots is in register with the slot 73 in the stationary mask. Since the movable mask is attached to the surface of the prism '49, the prism is mounted for bodily adjusting movement in directions edgewise of the stationary mask 70 and transverse to the axis of rotation, to enable any desired one of the slots 72a 72d to be brought into register with the slot 73, it being understood that the slot 73 is at least as wide as the widest of the slots in the movable mask.

The prism is mounted for such adjusting motion by reason of its securement to the bottom end of a rocking lever 79 which overlies the rear wall of the light shaft 45 above the fibre optic rod 48 and which extends generally lengthwise of the periscope body member 25. A screw 80 which is threaded into the periscope body has a shank portion which passes through the upper end portion of the rocking lever to provide a pivot therefor, the axis of said screw being transverse to the axis of rotation so that the lower end of the rocking lever can swing from side to side across the rear wall surface of the light shaft, carrying the prism with it.

The rocking lever is urged lightly against the rear surface of the light shaft, to confine it to swinging motion the'reacross, by means of a small expansion spring 83 that reacts between the rocking lever and the head of a screw 81. The screw 81, which has its axis parallel to that of the pivot screw 80, is threaded into the periscope body and extends through a transverse slot 82 in the rocking lever, a small distance above the prism.

The rocking lever is urged in one direction of its swinging motion by means of another expansion spring 84 that reacts between the rocking lever and one side wall surface of the light shaft. The limit of its swinging motion in said direction is defined by adjustment means comprising a threaded shaft or adjusting screw 85 which extends lengthwise along the light shaft, a follower or shoe 86 on the bottom of the threaded shaft, and aninclined cam surface 87 on the rocking lever, at the side thereof toward which it is biased by the spring 84, which cam surface is engaged by the shoe 86.

The spring 84 is piloted on a screw or pin 88 that projects toward the rocking lever from the sidewall surface of the light shaft, and is partly received in a sleeve 89 on the rocking lever.

The threaded shaft or adjusting screw is received in an internally threaded boss 90 on the periscope body member that projects into the light shaft. lts head, which is accessible near the 'top of the periscope body member, is provided with a screwdriver cross slot 91 or other means for facilitating rotation of it by which it is adjusted lengthwise along the light shaft. At its bottom it has a rotating connection with the shoe 86 by which the shoe is constrained to move axially with it, but the shoe is held against rotation with it by reason of the fact that the shoe has flat surface portions which slideably engage inner wall surface portions of the light shaft.

The cam surface 87 on the rocking lever diverges downwardly from a plane which lies on the axis 'of the pivot screw 80 and extends along the longitudinal centerline of the rocking lever. Hence as the threaded shaft 85'is turned in the direction to move the shoe downwardly, the shoe cooperates with the cam surface 87 to swing the rocking lever in the direction to increase the biasing force exerted by the spring 84. The different positions of adjustment of the rocking lever at which each of the several slots 72a72d are in register with the slot 73 in the stationary mask can be defined by suitable indicia, such as detent grooves in the cam surface 87 in which the shoe engages as it moves therealong.

Since the prism moves in a slightly arcuate path about the axis of the pivot screw, its bottom surface must be spaced a small distance from the top of the fibre optic rod 48, in order to insure the necessary clearance. For the same reason, the several slots 72a-72d in the movable mask on the prism must be in a slightly fan-shaped arrangement, with their longitudinal centerlines converging toward the axis of the pivot screw 84, as shown in FIG. 5.

From the foregoing description taken with the accompanying drawings it will be apparent that this invention provides simple and readily adjusted means for varying the sensitivity and selectivity of a spiral scanning mechanism for scanning bubble chamber photographs or the like along a spiral path, of the type comprising a rotating mirror and a periscope which rotates with the mirror and moves parallel to the axis of rotation.

We claim:

1. Mechanism for scanning an image such as a selected portion of a bubble chamber photograph along a spiral path that has its origin at a selected point on the image, and which mechanism comprises an objective focusable on the image, an elongated plane mirror mounted for orbital rotation about a defined axis with which the axis of light rays through the objective and said point on the image can be made to coincide, said mirror being inclined lengthwise obliquely to said axis and facing obliquely toward the same, a periscope having a surface which reflects light from the mirror and which is near the focal plane of the objective, and which periscope rotates with the mirror while moving parallel to said defined axis so that the reflecting surface always faces the mirror and scans lengthwise along it, and a light-responsive transducer operatively associated with the periscope to produce signals corresponding to changing values of light reflected from said surface in the course of the scan, said mechanism being characterized by:

means on the periscope for adjustably varying the sensitivity of the scanning mechanism in accordance with the density and contrast of an image to be scanned, said means comprising:

A. cooperating stationary and movable masks intercepting the light from the mirror, one behind the other, substantially on said focal plane;

1. the stationary one of said masks being fixed on the periscope and having a slot therein for the light, which slot extends along a plane that lies on said defined axis and is perpendicular to the mirror; and

2. the movable mask being apertured to define slots of different dimensions each of which can be brought into register with said slot in the stationary mask in consequence of adjusting movement of the movable mask to different positions along a defined path of adjustment; and

B. means mounting the movable mask on the periscope for adjusting movement relative thereto along said defined path.

2. The mechanism of claim 1, wherein the periscope comprises an elongated body having a substantially U-shaped cross section defining a slotlike light shaft extending lengthwise along it, reflector means, and a length of fibre optic in said light shaft extending lengthwise of the body with one of its ends adjacent to the reflector means, said mechanism being further characterized by:

A. a holder fixed to the periscope body near the reflector means, said holder being engaged with said end portion of the fibre optic to anchor the same and holding the stationary one of said masks in front of the reflector means;

B. the movable one of said masks being fixed on the reflector means and having thereina plurality of different width slots, each extending generally arallel to said axis; and C. said means mounting the mova le mask on the penscope comprising means mounting the reflector means for translatory adjusting movement in directions transverse to said defined axis to bring each in turn of said different width slots into register with said slot in the stationary mask.

3. The mechanism of claim 2, further characterized by said means mounting the reflector means on the periscope body for adjusting movement in directions transverse to said axis comprising: 1

A. a rocking lever which extends generally lengthwise along the periscope body in said light shaft and to one end portion of which the reflector means is fixed;

B. means pivotally securing the other end portion of said rocking lever to the periscope body so that the rocking lever is swingable to carry the reflector means in opposite directions transverse to said defined axis; and

C. means cooperating with the periscope body and the rocking lever to confine the latter in any selected position of swinging motion.

4. The mechanism of claim 3, further characterized by:

A. said rocking lever having a cam surface at one side thereof which is lengthwise inclined to a plane that extends lengthwise through the rocking lever and lies on the axis of said pivotal connection means; and

B. said means for confining the rocking lever in a desired position of its swinging motion comprising:

1. a threaded shaft extending lengthwise along the periscope body and having a connection therewith by which the shaft is rotatable to different positions of adjustment lengthwise of the periscope body;

2. follower means on one end of said threaded shaft constrained to move axially therewith and engageable with said surface on the rocking lever to define different limits of swinging motion of the rocking lever in one direction, depending upon the position of lengthwise adjustment of the threaded shaft; and

3. biasing means reacting between the periscope body and the rocking lever to urge the latter in its said one direction of swinging motion and thereby yieldingly maintaining said surface on the rocking lever engaged with the follower means.

5. Mechanism for scanning an image such as a selected portion of a bubble chamber photograph along a spiral path that has its origin at a selected point on the image, and which mechanism comprises an objective that can be focused on the image, an elongated plane mirror mounted for orbital rotation about a defined axis with which the axis of light rays through the objective and said point on the image can be made to coincide, said mirror being inclined lengthwise obliquely to said defined axis and facing obliquely toward the same, a periscope having a surface which reflects light from the mirror and which is near the focal plane of the objective, and which periscope rotates with the mirror while moving parallel to said defined axis so that the reflecting surface always faces the mirrot and scans lengthwise along it, and a light-responsive transducer operatively associated with the periscope to produce signals corresponding to changing values of reflected light in the course of the scan, said mechanism being characterized by means on the periscope for adjustably varying the sensitivity of the scanning mechanism in accordance with the density and contrast of an image to be scanned, said means comprising adjustable slot-defining means comprising a pair of substantially 

1. Mechanism for scanning an image such as a selected portion of a bubble chamber photograph along a spiral path that has its origin at a selected point on the image, and which mechanism comprises an objective focusable on the image, an elongated plane mirror mounted for orbital rotation about a defined axis with which the axis of light rays through the objective and said point on the image can be made to coincide, said mirror being inclined lengthwise obliquely to said axis and facing obliquely toward the same, a periscope having a surface which reflects light from the mirror and which is near the focal plane of the objective, and which periscope rotates with the mirror while moving parallel to said defined axis so that the reflecting surface always faces the mirror and scans lengthwise along it, and a light-responsive transducer operatively associated with the periscope to produce signals corresponding to changing values of light reflected from said surface in the course of the scan, said mechanism being characterized by: means on the periscope for adjustably varying the sensitivity of the scanning mechanism in accordance with the density and contrast of an image to be scanned, said means comprising: A. cooperating stationary and movable masks intercepting the light from the mirror, one behind the other, substantially on said focal plane;
 1. the stationary one of said masks being fixed on the periscope and having a slot therein for the light, which slot extends along a plane that lies on said defined axis and is perpendicular to the mirror; and
 2. the movable mask being apertured to define slots of different dimensions each of which can be brought into register with said slot in the stationary mask in consequence of adjusting movement of the movable mask to different positions along a defined path of adjustment; and B. means mounting the movable mask on the periscope for adjusting movement relative thereto along said defined path.
 2. follower means on one end of said threaded shaft constrained to move axially therewith and engageable with said surface on the rocking lever to define different limits of swinging motion of the rocking lever in one direction, depending upon the position of lengthwise adjustment of the threaded shaft; and
 2. the movable mask being apertured to define slots of different dimensions each of which can be brought into register with said slot in the stationary mask in consequence of adjusting movement of the movable mask to different positions along a defined path of adjustment; and B. means mounting the movable mask on the periscope for adjusting movement relative thereto along said defined path.
 2. The mechanism of claim 1, wherein the periscope comprises an elongated body having a substantially U-shaped cross section defining a slotlike light shaft extending lengthwise along it, reflector means, and a length of fibre optic in said light shaft extending lengthwise of the body with one of its ends adjacent to the reflector means, said mechanism being further characterized by: A. a holder fixed to the periscope body near the reflector means, said holder being engaged with said end portion of the fibre optic to anchor the same and holding the stationary one of said masks in front of the reflector means; B. the movable one of said masks being fixed on the reflector means and having therein a plurality of different width slots, each extending generally parallel to said axis; and C. said means mounting the movable mask on the periscope comprising means mounting the reflector means for translatory adjusting movement in directions transverse to said defined axis to bring each in turn of said different width slots into register with said slot in the stationary mask.
 3. The mechanism of claim 2, further characterized by said means mounting the reflector means on the periscope body for adjusting movement in directions transverse to said axis comprising: A. a rocking lever which extends generally lengthwise along the periscope body in said light shaft and to one end portion of which the reflector means is fixed; B. means pivotally securing the other end portion of said rocking lever to the periscope body so that the rocking lever is swingable to carry the reflector means in opposite directions transverse to said defined axis; and C. means cooperating with the periscope body and the rocking lever to confine the latter in any selected position of swinging motion.
 3. biasing means reacting between the periscope body and the rocking lever to urge the latter in its said one direction of swinging motion and thereby yieldingly maintaining said surface on the rocking lever engaged with the follower means.
 4. The mechanism of claim 3, further characterized by: A. said rocking lever having a cam surface at one side thereof which is lengthwise inclined to a plane that extends lengthwise through the rocking lever and lies on the axis of said pivotal connection means; and B. said means for confining the rocking lever in a desired position of its swinging motion comprising:
 5. Mechanism for scanning an image such as a selected portion of a bubble chamber photograph along a spiral path that has its origin at a selected point on the image, and which mechanism comprises an objective that can be focused on the image, an elongated plane mirror mounted for orbital rotation about a defined axis with which the axis of light rays through the objective and said point on the image can be made to coincide, said mirror being inclined lengthwise obliquely to said defined axis and facing obliquely toward the same, a periscope having a surface which reflects light from the mirror and which is near the focal plane of the objective, and which periscope rotates with the mirror while moving parallel to said defined axis so that the reflecting surface always faces the mirror and scans lengthwise along it, and a light-responsive transducer operatively associated with the periscope to produce signals corresponding to changing values of reflected light in the course of the scan, said mechanism being characterized by means on the periscope for adjustably varying the sensitivity of the scanning mechanism in accordance with the density and contrast of an image to be scanned, said means comprising adjustable slot-defining means comprising a pair of substantially flat members movable edgewise relative to one another and lying substantially on said focal plane, said members, in different positions of adjustment, defining elongated slots of different areas, each of which slots is symmetrical to a lengthwise centerline lying in a plane that lies on said defined axis and is perpendicular to the mirror. 